The production of human ES cells which can be either maintained in an undifferentiated state or directed to undergo differentiation into extraembryonic or somatic lineages in vitro allows for the study of the cellular and molecular biology of early human development, functional genomics, generation of differentiated cells from the stem cells for use in transplantation or drug screening and drug discovery in vitro.
In general, stem cells are undifferentiated cells which can give rise to a succession of mature functional cells. For example, a haematopoietic stem cell may give rise to any of the different types of terminally differentiated blood cells. ES cells are derived from the embryo and are pluripotent, thus possessing the capability of developing into any cell.
Much attention recently has been devoted to the potential applications of stem cells in biology and medicine. The properties of pluripotentiality and immortality are unique to ES cells and enable investigators to approach many issues in human biology and medicine for the first time. ES cells potentially can address the shortage of donor tissue for use in transplantation procedures, particularly where no alternative culture system can support growth of the required committed stem cell. However, it must be noted that almost all of the wide ranging potential applications of ES cell technology in human medicine-basic embryological research, functional genomics, growth factor and drug discovery, toxicology, and cell transplantation are based on the assumption that it will be possible to grow ES cells on a large scale, to introduce genetic modifications into them, and to direct their differentiation. Present systems fall short of these goals. Present systems for the growth of human ES cells include the use of Dulbecco's modified Eagle's medium as a basal media with the addition of amino acids and beta mercaptoethanol, serum supplementation, and embryonic mesenchymal feeder cell support. Growth under these conditions is not sufficient for many applications including scaleup of cultures and cloning of single cells, the latter being necessary for selection of transformants following genetic manipulation. Moreover, under present growth conditions, stem cells often follow a default pathway of differentiation into an epithelial cell type that grows either as flat squamous cells attached to the surface of the dish, or in cysts. It is likely that this form of differentiation represents extraembryonic endodermal differentiation and it is postulated that this cell type resembles the primary yolk sac cells of the primate embryo. As noted previously, conditions in which this form of differentiation predominates are unfavourable for differentiation of ES cells into desired somatic cell types. It would be desirable to control the differentiation pathways to maintain the cells in an undifferentiated state and direct the differentiation to the type of cell when required. Prior to differentiation, genetic manipulation of the ES cells may be conducted. However, it has been difficult to maintain the cells in the undifferentiated stem cell stage and prevent a default pathway to extraembryonic differentiation. Where somatic lineages are desired, a means of regulating against extraembryonic differentiation of ES cells is an important aspect in governing the fate of the cell. More importantly, greater control of the differentiation process is achieved when various stages of the differentiation process are regulated for instance, at a progenitor cell stage.
Accordingly, it is object of the present invention to overcome or alleviate some of the problems of the prior art and to achieve some regulation of the differentiation process of ES cells to somatic or extraembryonic lineages.